Frequency of terahertz electromagnetic wave ranges from 0.1 THz to 10 THz and its wavelength ranges from 30 micrometers to 3 millimeters. That is, the frequency range of terahertz electromagnetic wave is a boundary range between light wave and radio wave. Accordingly, terahertz electromagnetic wave passes through a substance like radio wave and travels in a straight line like light. Information on structures and behaviors of molecules can be obtained in the frequency range of terahertz electromagnetic wave, and therefore applications of terahertz electromagnetic wave in a various fields are anticipated. The fields include information and communication, nondestructive inspection, security field including detection of dangerous and hidden items, biomedical field including imaging of a malignant tumor, field of agriculture, field of industry, field of environment and field of space.
As a method for generating terahertz electromagnetic wave, a method in which nonlinear optical crystal is illuminated with femtosecond pulse laser to generate terahertz electromagnetic wave is known (JP2005-99453A, for example). When nonlinear optical crystal is illuminated with such a strong light as that of femtosecond pulse laser, polarization develops and vanishes in the nonlinear optical crystal and these phenomena are substantially identical with generation of pulsed current. As a consequence, the nonlinear optical crystal generates terahertz electromagnetic wave.
However, in the frequency range of terahertz electromagnetic wave, refractive index of a substance becomes higher because of dispersion. Accordingly, many components are reflected or absorbed by the substance when terahertz electromagnetic wave passes through the substance and therefore output of terahertz electromagnetic wave will be reduced. In order to prevent reduction in output of terahertz electromagnetic wave, an attempt that a thin film is provided on a surface of nonlinear optical crystal to reduce reflection on the surface has been made. However, efficiency of energy conversion into terahertz electromagnetic wave has not been remarkably enhanced because of the absence of a thin film material which is effective in the frequency range of terahertz electromagnetic wave and because of the existence of dispersibility of the thin film itself.
It is known that some materials easily transmit terahertz electromagnetic wave in a specific frequency band. An inspection system in which prohibited drugs, explosive items and the like are detected using the above-described feature, has been studied. Thus, in order to detect a specific substance, it is required to generate terahertz electromagnetic wave in the frequency band which is determined by the specific substance with energy conversion efficiency as high as possible.
Accordingly, there is a need for a terahertz electromagnetic wave generating element which generates terahertz electromagnetic wave in the predetermined frequency band with energy conversion efficiency as high as possible.